1. Field of the Invention
The present invention generally relates to a delay interferometer and a demodulator. More specifically, the present invention relates to a delay interferometer to be used to demodulate differential phase shift keying signal in an optical fiber communication, particularly in an optical fiber communication utilizing Dense Wavelength Division Multiplexing (DWDM). Further, the present invention relates to a demodulator including the delay interferometer.
Priority is claimed on Japanese Patent Application No. 2006-133708, filed May 12, 2006, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
The recent rapid development of the Internet needs to realize a high speed and large capacity network system by an optical fiber communication system that is configured to transmit optical signals of information through optical fibers, instead of electrical signals. The optical fiber communication system has already been developed and practiced. Dense Wavelength Division Multiplexing (DWDM) has been attracted to improve the high speed and large capacity of the optical fiber communication system. The DWDM is a technology which multiplexes multiple wavelength-different optical signals on a single optical fiber by utilizing the phenomenon that wavelength-different optical signals are not interfered with each other.
In the DWDM optical fiber communication system, typically, an optical signal is modulated by Differential Phase Shift Keying (DPSK) or Differential Quadrature Phase Shift Keying (DQPSK). The modulated optical signal is transmitted via the optical fiber to a demodulator that includes a delay interferometer. The modulated optical signal is demodulated by the demodulator.
Japanese Unexamined Patent Application, Publication, No. 2004-516743 discloses a conventional demodulator which demodulates DQPSK-modulated optical signal that has been modulated by Differential Quadrature Phase Shift Keying (DQPSK) in the DWDM optical fiber communication system.
FIG. 5 is a block diagram illustrating the configuration of the above-described conventional demodulator. The conventional demodulator 60 includes first and second optical fibers 61 and 62, first and second Mach-Zehnder interferometers 63 and 64, and first and second balanced photodetectors 65 and 66. The first and second optical fibers 61 and 62 provide first and second split paths, respectively. The conventional demodulator 60 is optically coupled to an optical fiber F that transmits a DQPSK-modulated signal. The first and second optical fibers 61 and 62 connect the optical fiber F to the first and second Mach-Zehnder interferometers 63 and 64, respectively. The first and second Mach-Zehnder interferometers 63 and 64 are realized by optical waveguides. The DQPSK-modulated signal is split into first and second split DQPSK-modulated optical signals which will be transmitted via the first and second optical fibers 61 and 62, respectively.
The first optical fiber 61 transmits the first split DQPSK-modulated optical signal to the first Mach-Zehnder interferometer 63. The second optical fiber 62 transmits the second split DQPSK-modulated optical signal to the second Mach-Zehnder interferometer 64.
The first balanced photodetector 65 includes first and second photodetectors 65a and 65b. The second balanced photodetector 66 includes third and fourth photodetectors 66a and 66b. 
The first Mach-Zehnder interferometer 63 includes first and second optical waveguides 63a and 63b, a first optical coupler 63c, third and fourth optical waveguides 63d and 63e. The first optical waveguide 63a is longer in optical path length by ΔL1 than the second optical waveguide 63b. The first split DQPSK-modulated optical signal is transmitted via the first optical fiber 61. The first split DQPSK-modulated optical signal is then further split into first and second sub-split DQPSK-modulated optical signals. The first and second optical waveguides 63a and 63b respectively transmit the first and second sub-split DQPSK-modulated optical signals to the first optical coupler 63c. 
The optical path length difference ΔL1 between the first and second optical waveguides 63a and 63b is set so that the first sub-split DQPSK-modulated optical signal having been transmitted via the first optical waveguide 63a does have a delay from the second sub-split DQPSK-modulated optical signal having been transmitted via the second optical waveguide 63b. The delay corresponds to one period of modulation rate or the symbol period. Additionally, a voltage application device is provided to apply a voltage across the second optical waveguide 63b, thereby applying an electric field to the second sub-split DQPSK-modulated optical signal being transmitted via the second optical waveguide 63b. The level of voltage to be applied across the second optical waveguide 63b is set so as to provide the second sub-split DQPSK-modulated optical signal with a π/4 phase shift. The voltage application device is not illustrated.
The first optical coupler 63c is optically coupled to the first and second optical waveguides 63a and 63b. The first optical coupler 63c is configured to optically couple or combine the first and second sub-split DQPSK-modulated optical signals together, thereby generating a first coupled signal.
The third and fourth optical waveguides 63d and 63e are optically coupled to the first optical coupler 63c. The third optical waveguide 63d transmits the first coupled signal and emits it to the first photodetector 65a of the first balanced photodetector 65. The fourth optical waveguide 63e transmits the first coupled signal and emits it to the second photodetector 65b of the first balanced photodetector 65. The third and fourth optical waveguides 63d and 63e have the same optical path length.
The second Mach-Zehnder interferometer 64 includes fifth and sixth optical waveguides 64a and 64b, a second optical coupler 64c, seventh and eighth optical waveguides 64d and 64e. The fifth optical waveguide 64a is longer in optical path length by ΔL1 than the sixth optical waveguide 64b. The second split DQPSK-modulated optical signal is transmitted via the second optical fiber 62. The second split DQPSK-modulated optical signal is then further split into third and fourth sub-split DQPSK-modulated optical signals. The fifth and sixth optical waveguides 64a and 64b respectively transmit the third and fourth sub-split DQPSK-modulated optical signals to the second optical coupler 64c. 
The optical path length difference ΔL1 between the fifth and sixth optical waveguides 64a and 64b is set so that the third sub-split DQPSK-modulated optical signal having been transmitted via the fifth optical waveguide 64a does have a delay from the fourth sub-split DQPSK-modulated optical signal having been transmitted via the sixth optical waveguide 64b. The delay corresponds to the symbol period. Additionally, a voltage application device is provided to apply a voltage across the sixth optical waveguide 64b, thereby applying an electric field to the fourth sub-split DQPSK-modulated optical signal being transmitted via the sixth optical waveguide 64b. The level of voltage to be applied across the sixth optical waveguide 64b is set so as to provide the fourth sub-split DQPSK-modulated optical signal with a −π/4 phase shift. The voltage application device is not illustrated.
The second optical coupler 64c is optically coupled to the fifth and sixth optical waveguides 64a and 64b. The second optical coupler 64c is configured to optically couple or combine the third and fourth sub-split DQPSK-modulated optical signals together, thereby generating a second coupled signal.
The seventh and eighth optical waveguides 64d and 64e are optically coupled to the second optical coupler 64c. The seventh optical waveguide 64d transmits the second coupled signal and emits it to the third photodetector 66a of the second balanced photodetector 66. The eighth optical waveguide 64e transmits the second coupled signal and emits it to the fourth photodetector 66b of the second balanced photodetector 66. The seventh and eighth optical waveguides 64d and 64e have the same optical path length.
The first balanced photodetector 65 includes the first and second photodetectors 65a and 65b that are configured to generate first and second electrical signals, depending upon the intensity of the first coupled optical signal. Namely, the first and second electrical signals each indicate the intensity of the first coupled optical signal. The first balanced photodetector 65 performs a balancing process for the first and second electrical signals, thereby generating a first demodulated signal “x”.
The second balanced photodetector 66 includes the third and fourth photodetectors 66a and 66b that are configured to generate third and fourth electrical signals, depending upon the intensity of the second coupled optical signal. Namely, the third and fourth electrical signals each indicate the intensity of the second coupled optical signal. The second balanced photodetector 66 performs a balancing process for the third and fourth electrical signals, thereby generating a second demodulated signal “y”.
As described above, the conventional demodulator 60 is configured to demodulate the DQPSK-modulated optical signal. The conventional demodulator 60 includes the first and second Mach-Zehnder interferometers 63 and 64. The first Mach-Zehnder interferometer 63 has the two waveguides that are configured to provide the DQPSK optical signal with the π/4 phase shift and the time delay which corresponds to the symbol period. The second Mach-Zehnder interferometer 64 has the two waveguides that are configured to provide the DQPSK optical signal with the −π/4 phase shift and the time delay which corresponds to the symbol period. The first and second demodulated signals “x” and “y” are used as a binary digit signal.
Another conventional demodulator may be configured to demodulate a DPSK-modulated optical signal. The other conventional demodulator needs to include a single optical fiber, a single Mach-Zehnder interferometer, and a single balanced photodetector. It is unnecessary to provide the DPSK-modulated optical signal with any phase shift.
The conventional demodulator uses the optical waveguide Mach-Zehnder interferometer, which may cause the following disadvantages.
The first disadvantage is that a highly accurate temperature control for the modulator is necessary to stabilize the performances of the optical waveguide or waveguides. Realizing the highly accurate temperature control may increase the cost and the dimensions of the demodulator.
The second disadvantage is that the conventional demodulator has poor mechanical stress stability, thereby causing variation of the performances of the optical waveguides.
The third disadvantage is that the Mach-Zehnder interferometer is connected via an additional optical fiber to the balanced photodetector even illustration of the additional optical fiber is omitted in FIG. 5. This connection structure may cause an undesired delay of optical signal having been transmitted from the Mach-Zehnder interferometer to the balanced photodetector.
The fourth disadvantage is that it is difficult to ensure the reproductivity of the ±π/4 phase shifting process for the DQPSK optical signal.
The conventional demodulator has poor demodulation performances in stability and accuracy.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved apparatus and/or method. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.